This invention relates to a method of making hypophosphorous acid (HPA) using electrodialytic water splitting. It also relates to an improved method of making sodium hypophosphite wherein the sodium hypophosphite is used in an electrodialytic water splitting process to make hypophosphorous acid and the depleted sodium hypophosphite from the electrodialytic water splitting process is used to adjust the pH in the process for making sodium hypophosphite.
Electrodialytic water splitting is a process in which a solution of a salt is subjected to a direct current, decomposing water and causing the anions and cations to pass through anion exchange and cation exchange membranes, respectively, forming an acid and a base in separate compartments. For example, if a solution of sodium chloride is placed between a cation exchange membrane and an anion exchange membrane and is subjected to a direct current, hydrochloric acid will form on the other side of the anion exchange membrane and sodium hydroxide will form the other side of the cation exchange membrane. The concentration of the acid formed is a function of the current density, but the higher the concentration of the acid, the lower will be the current efficiency.
There is also a relationship between the strength of the acid (i.e., how strongly it dissociates to liberate H.sup.+) and the concentration of the acid that can be obtained at a particular current efficiency. At the same current efficiency, it is possible to obtain higher concentrations of weak acids than of strong acids. The relationship between the strength of the acid and the concentration of the acid that can be obtained at a given current efficiency can be found in an article by K. N. Mani titled, "Electrodialysis Water Splitting Technology," Journal of Membrane Science, 58 (1991) pps. 117-138 at page 122. (This article is hereinafter referred to as "Mani, 1991".) In that article it states that at a current efficiency of 80% or higher and a current density of 100 mA/cm.sup.2 the concentration of a strong acid that can be obtained is about 1 normal and the concentration of a weak acid that can be obtained is about 3 to about 6 normal. Because hypophosphorous acid is a strong monobasic acid, the normality and molarity are equal values (i.e., 1N H.sub.3 PO.sub.2 =1M H.sub.3 PO.sub.2).
Hypophosphorous acid is now produced by the acidification of sodium hypophosphite. For example, one can load a cation exchange resin with hydrogen ions and pass a solution of sodium hypophosphite over the resin so that the sodium ion is exchanged for the hydrogen ion and hypophosphorous acid is produced. Until now, the production of hypophosphorous acid by electrodialytic water splitting has not been suggested or attempted. One possible reason for this is that the readily available references for HPA describe it as a strong acid. As a strong acid, the concentration of HPA that could be produced at a given current efficiency would be low, requiring a choice between high power consumption to make a high concentration of acid and high energy consumption to evaporate water from a low concentration of acid. Van Wazer, Phosphorus and Its Compounds, Vol I., page 359 (1958), the definitive reference on phosphorous chemistry, describes HPA as a strong monobasic acid with a dissociation constant of 8.0.times.10.sup.-2 (pKa=1.1). On the other hand, K. Mani in WO 92/11080 defines a weak acid for the purpose of electrodialytic water splitting as one with a pKa of 3 or greater, but generally less than 11. The electrodialytic water splitting of sodium hypophosphite would therefore be expected to produce hypophosphorous acid having a concentration of about 1 normal at a current efficiency of 80%. Another reason, relating to production in conventional electrolytic membrane cells, may be due to product quality concerns associated with oxidation of the hypophosphorous acid at the anode to produce phosphite anion contamination of the product.